e1000e: cleanup: remove e1000_get_cable_length()

Remove the function e1000_get_cable_length() and replace the two calls
to it with the same function pointer that it would call.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>